Several national governments, including the United States (U.S.), are presently developing a terrestrial position determination system, referred to generically as a global positioning system or GPS. Global positioning systems include a number of satellites placed in orbit around the Earth. The GPS satellites are designed to transmit electromagnetic signals. From these electromagnetic signals, the absolute or terrestrial position, that is, position with respect to the Earth's center, of any GPS receiver at or near the Earth's surface can ultimately be determined.
The U.S. government has designated its global positioning system, NAVSTAR. It is anticipated that the NAVSTAR GPS will be declared operational by the U.S. government in 1993. The NAVSTAR GPS envisions four orbiting GPS satellites in each of six orbits. A total of 24 GPS satellites will be in orbit at any given time with twenty-one (21) GPS satellites in operation and three (3) satellites serving as spares. The six (6) orbits will have mutually orthogonal planes with respect to the Earth. The orbits will be neither polar nor equatorial. Furthermore, the GPS satellites will orbit the Earth approximately every twelve (12) hours.
Using the NAVSTAR GPS, the relative distance or range from the orbiting GPS satellites with respect to any GPS receiver can be determined by the electromagnetic signals. The relative distance from a satellite to the receiver is commonly referred to as a pseudorange.
In the NAVSTAR GPS, electromagnetic signals are continuously transmitted from all of the GPS satellites at a single carrier frequency. However, each of the GPS satellites has a different gold code, thereby allowing for differentiation of the signals. In the NAVSTAR GPS, the carrier frequency is modulated using a pseudorandom signal which is unique to each GPS satellite. Consequently, the orbiting satellites in the NAVSTAR GPS can be identified when the carrier frequencies are demodulated.
Furthermore, the NAVSTAR GPS envisions two modes of modulating the carrier wave signal using pseudorandom number (PRN) signals. In one mode, referred to as the "coarse/acquisition" or "C/A" mode, the PRN signal is a gold code sequence having a chip rate of 1.023 MHz. The gold code sequence is a conventional pseudorandom sequence which is known in the art. A chip is one individual pulse of the pseudorandom code. The chip rate of a pseudorandom code sequence is the rate at which the chips in the sequence are generated. Consequently, the chip rate is equal to the code repetition rate divided by the number of members in the code. Accordingly, with respect to the C/A mode, there exists 1.023 chips in each gold code sequence and the sequence is repeated once every millisecond. Use of the 1.023 MHz gold code sequence from four GPS satellites enables the terrestrial position of an Earth receiver to be determined with an approximate accuracy of 60-300 meters.
The second mode of modulation in the NAVSTAR GPS is commonly referred to as the "precise" or "protected" (P) mode. In the P mode, the pseudorandom code has a chip rate of 10.23 MHz. Moreover, the P mode sequences are extremely long, so that the sequences repeat no more than once every 267 days. As a result, the terrestrial position of any Earth receiver can be determined to within an approximate accuracy of 16-30 meters.
However, the P mode sequences are held in secrecy by the U.S. government and are not made publicly available. In other words, the P mode is intended for use only by Earth receivers authorized by the U.S. government.
For more a detailed discussion on the NAVSTAR GPS, see Parkinson, Bradford W. and Gilbert, Stephen W., "NAVSTAR: Global Positioning System--Ten Years Later," Proceedings of the IEEE, Vol. 71, No. 10, October 1983, which is incorporated herein by reference. For a detailed discussion of a vehicle positioning/navigation system which uses the NAVSTAR GPS, see U.S. patent application Ser. No. 07/628,560, entitled "Vehicle Position Determination System and Method, filed Dec. 3, 1990, which is incorporated herein by reference.
Pseudoranges are determined by measuring the propagation time delays between the time of transmission and the time of reception of the electromagnetic signals. In the NAVSTAR GPS, the electromagnetic signals are encoded continuously with the time at which the signals are transmitted from the GPS satellites. The transmission time can be subtracted from the reception time to determine a time delay. From the calculated time delay, the pseudorange can be accurately derived by multiplying the propagation time delay by the speed of transmission (approximately 2.99792458.times.108 m/s).
The absolute position of any GPS receiver can be determined using the pseudoranges of at least three GPS satellites via simple geometric theory involving triangulation methods. The accuracy of the terrestrial position estimate is partially dependent upon the number of GPS satellites that are sampled. Using more GPS satellites in the computation can increase the accuracy of the terrestrial position estimate.
Conventionally, four GPS satellites are sampled to determine each terrestrial position estimate because of errors contributed by circuit clock differentials among the GPS receivers and the GPS satellites. Clock differentials may be as large as several milliseconds. Ideally, there are more than four satellites "visible" to the receiver. That is, the receiver is able to receive signals from more than four satellites. When there are more than four satellites visible, the four satellites which will give the most accurate position are used. The GPS receiver may be adapted to select satellites based on its own criteria or the user may conversely select the satellites according to different criteria. The selected satellites are normally referred to as a constellation.
However, there are presently some periods of time when less than four or possibly only one satellite is in view. This may be due to natural or manmade obstacles, for example, a building or a cliff, that block the signals or this may be due to satellite malfunction or selective availability. Selective availability or SA refers to the government's selective shutting down or commanding a satellite to transmit bad data in the C/A mode, for example offsetting the orbit parameters or desynchronizing the satellite clocks. This leaves the P mode still operational to users authorized by the government. Since only government authorized receivers can use the P mode, it is desirable to provide positioning capability using the C/A mode during this period.
The present invention is adapted to overcome one or more of the problems as set forth above.